This invention relates to soldering methods and more particularly to a method of soldering printed circuit boards while eliminating the step of post cleaning.
As is well known, soldering is a method of attaching one metallic material to another. it has been used in the electronics industry for some time.
In regard to the fabrication of printed circuits, soldering is accomplished by a process sometimes referred to as dip soldering. Dip soldering consists of applying a flux, usually a rosin-alcohol mixture to the circuit pattern and then placing the board in contact with the molten solder. Slight agitation of the board assures good fillets around the wire leads.
In present techniques, the circuit board with its components assembled on one side of the board, has the conductor pattern pass through the crest of a wave of molten solder. In this technique referred to as wave soldering, all junctions are soldered as the board progresses through the wave. The flux, board temperatutre, solder temperature, and immersion time are interrelated and are adjusted for best results. Essentially, long exposure to hot solder is detrimental to the insulating material and to the adhesive that joins the copper foils to the insulation.
After solder dipping, the residual flux should be removed by a suitable solvent. One has to be sure that the solvent is compatible with the materials used in the components parts mounted on the board. Certain solvents frequently dissolve cements or plastics and marking inks and can further cause severe stress cracking of plastics.
Examples of soldering techniques and considerations can be had by reference to a book entitled "Printed Circuit Boards for Microelectronics" by J. A. Scarlett, published by Van Nostrand Reinhold Co., New York (1970).
As above indicated, the problems of cleaning printed circuit assemblies after wave soldering are considerable. For example, certain electronic components are extremely sensitive to the effects of chloro or fluoro hydrocarbons which are used as solvents. In regard to such uses, some of the reactions are immediately apparent, while others take a long time to provide an adverse reaction. It is thus apparent that the cleaning of circuit boards after wave soldering by the use of solvents creates extreme problems. A further difficulty is that the solvents employed such as hydrocarbons present health hazards to the user, as they are environmental pollutants.
it is further noted that post cleaning of printed circuit boards requires additional manufacturing time as well as additional materials. Hence, as can be ascertained, it would be extremely desirable to perform wave soldering of printed circuit boards without the necessity of post cleaning the boards with a solvent. To achieve such an operation, one would experience an overall cost reduction attendant with the following advantages:
(1) Elimination of solvents resulting in the elimination of a health hazard and a manufacturing component;
(2) Elimination of the manufacturing process time in a soldering operation by omission of the post cleaning step;
(3) Elimination of an additional operation of hand soldering components which are susceptible to solvent damage;
(4) Elimination of delayed damage to components not discovered until after prolonged operation of the circuit board;
(5) Elimination of wash-off of certain paint and ink markings on components;
(6) Permitting less restrictive material parameters for component construction which allows a wider selection of components for printed circuit boards and hence more economical procurement.
The advantages of the method to be described are as indicated above, while further enhancing the final product by the provision of an encapsulant type coating. The coating may serve to enhance the insulation value of the substrate surface between components and between traces and hence, provide a possible improvement in resistance of the assembly against normal or mildly pollutant atmospheres.
It will thus be shown that with the proper selection of processing materials and applied techniques, printed circuit assemblies are manufactured without recourse to post soldering cleaning.